Background: Sensitive/Less Sensitive assays (S/LS) to distinguish between recent and established HIV infections have been successfully applied to identify recently infected persons and determine HIV incidence. However, all currently used S/LS assays require laboratory instrumentation which limit their applicability in resource poor countries where stable electricity and laboratory infrastructure are lacking. Similarly, these assays require a high degree of technical expertise that is limited in a majority of laboratories throughout the world, and the cost of these tests is relatively high. More technically simple, cost-effective, and robust assays are needed to promote the use of S/LS assays by a larger range of users and for use where non-B HIV clades exist. The Serodia HIV-1/HIV-2 particle agglutination assay (PA), a simple HIV assay that is used globally and can be performed by non-HIV specialists, has been modified by our group as an S/LS assay and shown to possess a high positive predictive value for detecting truly recently infected individuals, and a 97% sensitivity for classifying recent and established infection specimens when tested on seroconversion panels for clade B HIV viruses at cost 30 times lower than other S/LS tests. Methods: The PA S/LS test will be calibrated against seroconversion panels from individuals in India and Nigeria where HIV clades C and A/G predominate, respectively, for expanding its use to countries where non-B clade viruses exist. A total of 70 seroconversion panels from the two countries will be used to calibrate the PA S/LS test for these HIV clades by varying sample dilutions and assay parameters. Selection of the parameters that best identifies recent infection will be accomplished through receiver operator curve analysis, and the days post seroconversion threshold will be determined for each clade for the purposes of (1) correctly classifying recently infected individuals with a high predictive value for enrollment in early intervention programs, and (2) for estimating the HIV incidence in a variety of high risk populations. Subsequently, in year 2, the calibrated test will be used to classify recently infected persons and to determine the HIV incidence in Abuja, Nigeria by testing 1,300 banked, seropositive specimens. Specifically, the incidence density will be estimated from 10,000 screening samples and surveillance data from antenatal clinic attendees, commercial sex workers, motorcycle taxi drivers, and STD clinic attendees. Outcome: The PA S/LS test will be calibrated for application in a wider range of countries, for use with a broader range of HIV clades, and will offer substantial advantages over current tests for use in resource-limited countries. Additionally, the PA S/LS test will be applied to measure the incidence of HIV in Nigeria in order to provide information to substantiate the need for targeted, early intervention counseling and vaccine preparedness initiatives to help in preventing transmission by recently infected, highly infectious individuals. [unreadable] [unreadable] [unreadable]